1. Field of the Invention
The present invention relates to the manufacture of refractory compounds. More particularly, the invention relates to the production of the nitrides of the refractory metals, rare earth metals, actinide series metals, and combinations thereof. In its particularly preferred embodiment the invention provides a method for producing a mononitride of thorium, uranium, plutonium, or mixtures thereof, which mononitride is recovered in the form of ultrafine particles ideally suited for sintering to form a dense compact fuel element for a nuclear reactor.
2. Prior Art
It is well known that metals such as vanadium, titanium, thorium, niobium, zirconium, hafnium, tungsten, molybdenum, tantalum, uranium, plutonium, and silicon have refractory compounds including the carbides, nitrides, borides, phosphides, and the like. These compounds are difficult to manufacture as their melting points are high, for example, in the region of 2000.degree.-3000.degree. C. The compounds have many purposes which depend upon their high melting points and chemical and physical characteristics. The compounds of uranium, plutonium, and thorium are of particular interest in view of their potential use as nuclear fuel materials.
Indeed, the excellent physical and nuclear properties of the nitrides of such materials mark them as an excellent fuel for use in a high temperature, high power density nuclear reactor. In the case of uranium nitride, for example, it can be substituted for UO.sub.2 in a fuel element and occupy about 30% less volume with an equivalent uranium content. Its high thermal conductivity (on a par with UC), high melting point, and acceptable chemical compatibility are further recommendations for its use as a nuclear fuel. However, the acceptance of this material as a fuel for current commercial power reactors is at least partly contingent upon the overall economy associated with its production and fabrication into a fuel element. One of the principal cost factors involved is the fabrication of the nitride into a densified compact. Specifically, to form a dense compact of the material, it is essential, if it is to be used as a fuel, that it be substantially pure and all in the mononitride form, and additionally, have an ultrafine particle size.
Considerable work has been done in attempting to develop a viable process for the preparation of ultrafine particles of uranium mononitride. In U.S. Pat. No. 2,544,277 it is suggested that uranium first be reacted with hydrogen until it is substantially all converted to a uranium hydride. Thereafter, the uranium hydride is reacted with ammonia or nitrogen at a temperature of from about 200.degree. C. to 400.degree. C. to form a uranium nitride, which subsequently is calcined at about 1400.degree. C. to form a compound corresponding approximately to uranium mononitride.
In U.S. Pat. No. 3,180,702 it is suggested that uranium nitride be formed by reacting finely divided uranium particles with nitrogen in the presence of hydrogen at a temperature between 450.degree. C. and 1200.degree. C. Thereafter, the excess nitrogen in the product uranium nitride is removed by heating the uranium nitride in a vacuum at a temperature above about 1000.degree. C. to form the mononitride.
U.S. Pat. No. 3,322,510 discloses yet another process for the preparation of certain metallic nitrides. Prior to forming the nitride a surface hydride coating is provided on the selected metal by reacting it with a small quantity of hydrogen, and thereafter the hydride-coated metal is reacted with nitrogen at elevated temperatures to form the nitride.
In East German Pat. No. 30,160 there is disclosed a process for the production of the nitrides of uranium and plutonium utilizing a molten salt (alkali metal halide) bath. In the process disclosed therein, a uranium halide (UCl.sub.4) is reacted with ammonia to form a uranium nitride and a byproduct of ammonium chloride. Thus, the process requires that the uranium metal first be reacted or treated to form a uranium halide and then introduced into the bath for reaction with the ammonia. Further, the process also results in an unneeded byproduct, i.e., ammonium chloride.